1. Field of the Invention
The present invention relates to an optical pickup apparatus, a signal generating method, and an optical disk apparatus and, in particular, relates to an optical pickup apparatus enabling application to optical disks having a plurality of recording layers, a signal generating method of generating a reproduced signal from an output signal of the optical pickup apparatus, and an optical disk apparatus suitably used for an optical disk apparatus enabling application to optical disks having a plurality of recording layers.
2. Description of the Related Art
In recent years, the capacity of optical disks is rapidly increasing. The increase in the capacity of an optical disk is made possible by disposing a plurality of recording layers within a single disk. For example, multilayer-type disks including two recording layers on a single side have been commercialized for DVD (Digital Versatile Disk) or Blu-ray disks. When the number of recording layers is three or more, further increase in the capacity of the optical disk is enabled.
However, when a plurality of recording layers are disposed within the disk, a problem occurs that a reflected light from recording layers other than a reproduction target is incident on a photodetector, which causes deterioration in the reproduced signal. That is, a problem of interlayer crosstalk resulting from a stray light arises. Thus, when disposing a plurality of recording layers within a single disk, there needs a configuration for smoothly inhibiting and removing the stray light.
Herein, a method of removing the stray light includes that which uses a pinhole. In this method, a pinhole is disposed at a convergence position of the signal light. According to this method, a part of the stray light is intercepted by the pinhole, and therefore, it is possible to lower the unnecessary stray light component incident on the photodetector.
A further method of removing a stray light is a method of disposing a forward light shielding plate and a rear light shielding plate before a photodetector. In this method, before and after the focus point of the signal light, the forward light shielding plate and the rear light shielding plate are disposed. According to this method, only the signal light passes between the both light shielding plates, and the stray light is shielded by the both light shielding plates. Furthermore, another method of removing a stray light is a method using a polarizing optical element. In this method, an optical system is so set that the signal light and the stray light are incident on the polarizing optical element in different polarization directions. According to this method, only the signal light passes through the polarizing optical element, and thus, it may be possible to remove the unnecessary stray light components incident on the photodetector.
However, in the method in which a pinhole is disposed before the photodetector, in addition to the reflected light from reproduction-target recording layers, the stray light passes through the pinhole and reaches the photodetector. In this case, separation between recording layers is poor, and when the interval between layers is small, the amount of stray light passing through the pinhole increases, and thus results in a problem that it is difficult to properly remove interlayer crosstalk.
Furthermore, when the method of disposing a forward light shielding plate and a rear light shielding plate is used, half the reflected light from the reproduction-target recording layer is shielded. Thus, a problem arises that the use efficiency of the reflected light is decreased by about 50%. When the method employing a polarizing optical element is used, many optical components are required in order to remove the stray light, and thus, there occurs a problem that cost rises.